FST 400- Properties of foods and processing theory

Liquids, gases and some solids (for example powders and particulate materials) are termed ‘fluids’ and can flow without disintegration when a pressure is applied to them. In contrast, solids deform when pressure is applied to them (Section 1.1.4). In this section, the properties of fluids and solids that are relevant to both the design of food processes and the quality of processed food are described. More detailed mathematical treatments and derivations of formulae used in food engineering calculations are given in a number of texts including Toledo (1999), Lewis (1990), Brennan et al. (1990) and Earle (1983). The transition from solid to fluid and back is known as a phase transition and this is important in many types of food processing (e.g. water to water vapour in evaporation and distillation (Chapter 13) and dehydration (Chapter 15); water to ice in freezing (Chapter 21) and freeze drying or freeze concentration (Chapter 22) or crystallisation of fats (Chapter 23)). Phase transition takes place isothermally at the phase transition temperature by release or absorption of latent heat, and can be represented by a phase diagram (for example Chapter 22, Fig. 22.1). A second type of transition, known as glass transition, takes place without the release or absorption of latent heat and involves the transition of a food to an amorphous glass state at its glass transition temperature. The transition is dependent on the temperature of the food, time, and the moisture content of the food. Examples of glass transition temperatures are given in Chapter 21 (Table 21.2). When materials change to glasses, they do not become crystalline, but retain the disorder of the liquid state. However, their physical, mechanical, electrical and thermal properties change as they undergo the transition. In their glassy state, foods become very stable because compounds that are involved in chemical reactions that lead to deterioration are immobilised and take long periods of time to diffuse through the material to react together. Details of nine key concepts underlying the relationship between molecular mobility and food stability are given by Fennema (1996). Processes that are significantly influenced by transition to a glassy state include aroma retention, crystallisation, enzyme activity, microbial activity, non-enzymic browning, oxidation, agglomeration and caking.